First, getting the board running from pre-packaged Ubuntu image is easy thanks to armhf.com:
http://www.armhf.com/index.php/boards/nitrogen6x-sabre/
Building a custom kernel ... you don't have to be a trailblazer here, just follow instructions:
Refer to http://www.eewiki.net/display/linuxonarm/i.MX6x+SABRE+Lite
I built some kernels from github.com/RobertCNelson/armv7-multiplatform.git
... but they choked on USB.
Instead I altered the procedure slightly to match the 3.9 kernel as found in the armhf.com image....
git clone https://github.com/RobertCNelson/imx-devel.git
cd imx-devel/
git checkout origin/v3.9.x-imx -b tmp
./build_kernel.sh
sudo mount /dev/sdb1 /media/tmp/
sudo cp -v imx-devel/deploy/3.9.11-imx5.zImage /media/tmp/boot/zImage
sudo tar xfv ./imx-devel/deploy/3.9.11-imx5-dtbs.tar.gz -C /media/tmp/boot/dtbs/
sudo tar xfv ./imx-devel/deploy/3.9.11-imx5-modules.tar.gz -C /media/tmp/
After completing the above steps, I rebooted the Sabre Lite and ended up with this:
ubuntu@ubuntu-armhf:~$ uname -a
Linux ubuntu-armhf 3.9.11-imx5 #1 SMP Thu Feb 27 12:22:07 EST 2014 armv7l armv7l armv7l GNU/Linux
